Conventional vapor compression systems typically include a compressor, a heat rejection heat exchanger, a heat absorption heat exchanger, and an expansion device disposed upstream of the heat absorption heat exchanger. Some systems further include a second expansion device downstream of the heat rejection heat exchanger, such as a suction modulation valve. These basic system components are interconnected by working fluid lines in a closed circuit.
Depending upon the characteristics of the working fluid in use in a particular application, a vapor compression system may be operated in either a subcritical mode or a transcritical mode. In vapor compression systems operating in a subcritical cycle, both the vapor heat rejection heat exchanger and the heat absorption heat exchanger operate at pressures below the critical pressure of the working fluid. Thus, in the subcritical mode, the vapor heat rejection heat exchanger functions as a working fluid condenser and the heat absorption heat exchanger functions as a working fluid evaporator.
However, in refrigerant vapor compression systems operating in a transcritical cycle, the vapor heat rejection heat exchanger operates at a refrigerant temperature and pressure in excess of the refrigerant's critical pressure, while the heat absorption heat exchanger operates at a refrigerant temperature and pressure in the subcritical range. Thus, in the transcritical mode, the vapor heat rejection heat exchanger functions as a working fluid gas cooler and the heat absorption heat exchanger functions an as a working fluid evaporator.
In vapor compression systems used in refrigeration applications, commonly referred to as refrigerant vapor compression systems, the working fluid is refrigerant. Refrigerant vapor compression systems charged with conventional refrigerants, such as for example, fluorocarbon refrigerants such as, but not limited to, hydro chlorofluorocarbons (HCFCs), such as R22, and more commonly hydro fluorocarbons (HFCs), such as R134a, R404A, and R407C, typically operate in the subcritical mode. “Natural” refrigerants, such as carbon dioxide, are also used in refrigerant vapor compression systems instead of HCFC or HFC refrigerants. Because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical mode.
Refrigerant vapor compression systems used in connection with transport refrigeration systems are generally subject to more stringent operating conditions than in air conditioning or commercial refrigeration applications due to the wide range of operating load conditions and the wide range of outdoor ambient conditions over which the refrigerant vapor compression system must operate to maintain product within the cargo space at a desired temperature. The desired temperature at which the cargo needs to be controlled can also vary over a wide range depending on the nature of cargo to be preserved. The refrigerant vapor compression system must not only have sufficient capacity to rapidly pull down the temperature of product loaded into the cargo space at ambient temperature, but also operate efficiently at low load when maintaining a stable product temperature during transport. Additionally, transport refrigerant vapor compression systems are subject to cycling between an operating mode and standstill mode, i.e., an idle state.